A multilayer optical film (MOF) comprises multiple layers of two or more materials, where these layers have differing indices of refraction. Such multilayer optical films can result in reflections of given bands of wavelengths, and are well known in theory and in practice, see for example U.S. Pat. No. 3,711,176 (Alfrey et al.), U.S. Pat. No. 4,162,343 (Wilcox, F. S. et al.), (Radford, J. A., Alfrey, T., Schrenk, W. J.; Reflectivity of iridescent coextruded multilayered plastic films; Polymer engineering and science, May, 1973, Vol. 13, No. 3, p. 216), (Macleod, H. A.; Thin-film optical filters, 3rd edition, Institute of Physics Publishing, 641 p.), and (Willey, R. R., Practical design and production of optical thin films, 2nd edition, Marcel Dekker, 2002, 547 pages). MOFs are widely used in applications that include tunable lasers, filters, waveguides, fiber-optics, optics and telecommunications, filters for fiber-optics communications, security devices and pigments, decorative coatings and films, among others.
MOFs may also be known as optical multilayers, Bragg stacks, distributed Bragg reflectors, Bragg gratings, or 1-dimensional photonic crystals. MOFs may be prepared in a number of ways, including coextrusion, physical vapor deposition (including sputtering and evaporation), chemical vapor deposition, dip-coating, liquid coatings, amongst others (see Ullmann's Encyclopedia of Industrial Chemistry—Thin Films, Vol. 36, pp. 519-587). They are used widely in the fields of optics and telecommunication, lasers, filters, waveguides, fiber-optics, security devices and pigments, decorative coatings and films, amongst others.
The following patent publications are referenced here as they relate to photonic crystals: PCT/CA2007/000236; PCT/CA2009/000273; PCT/CA2009/000745; PCT/CA2009/001052; CA2009/001170; PCT/CA2009/001672; PCT/CA2010/000293; PCT/CA2010/001604; PCT/CA2011/001254; PCTCA2011/001363; PCT/CA2012/000077; and PCT/CA 012/000517. The disclosures of these references are incorporated herein by reference in their entirety.
MOFs may be constructed from layers of two or more materials, most often layered on top of each other in a periodic arrangement. If the two or more materials have a difference in refractive index, light may be partially reflected at each boundary, and upon re-exiting the material the light waves might at least partially interfere with each other. If the light waves constructively interfere, these particular wavelengths will be reflected from the material in a mirror-like configuration. Destructive interference, on the other hand, results in light being transmitted through the material. Partial interference would cause effects intermediate these two cases. A MOF typically comprises periodically alternating layers of a relatively high refractive index material and a relatively low refractive index material. As incident light impinges on the MOF, a series of scattering events ensue whereby only a narrow band of wavelengths, typically centered about one single wavelength, λ, are coherently diffracted and reflect off the surface of the material. The normal incidence first-order central wavelength, λI, may be predicted by the Bragg equation (Wu et al., Small 2007, vol. 3 p. 1445):λI=2(n1d1+n2d2)  (equation 1)where n is the refractive index, and d is the geometric thickness of each layer 1 and 2.
Therefore, the reflection properties of a particular MOF depend primarily on the optical thickness of each layer, where the optical thickness is the refractive index multiplied by the geometric thickness (nd). The reflection of a MOF can be designed to fall in the UV, visible, or infrared regions by correctly choosing the optical thickness of the constituent layers. Furthermore, if the optical thickness of one or more layers can be changed in a controlled manner, the reflected color may also be changed in a controlled manner (see US Patent application 2011/0164308 A1, Arsenault A. C. et al.).
It should be noted that this equation governs the reflection of a particular band of wavelengths, which can be used advantageously in some applications. While a given band of wavelengths is reflected, the remainder of the wavelengths are allowed to pass through the material relatively unimpeded, depending on the scattering/absorption properties of the material. Therefore, such a material may be viewed in reflection which results in a particular color perception. Such a material may also be viewed in transmission, which would give a complimentary color to the reflected color. For example, a material reflecting green light would give a magenta color in transmission, whereas a material reflecting blue light would give a yellow color in transmission.
The intensity of the reflectance band at λ in air is given by the following expression (see (Macleod, H. A.; Thin-film optical filters, 3rd edition, Institute of Physics Publishing, 641 p.)):R=[(1−Y)/(1+Y)]2×100(%)  (equation 2)Y=(n2/n1)N−1(n22/ns)  (equation 3)where n2, n1, and ns are the refractive indices of the high-index material, the low-index material, and a substrate, respectively, and N is the number of layers in the MOF. As can be seen, the reflectance, R, increases with increasing values of the refractive-index contrast ratio, n2/n1, as well as with the number of layers in the stack (N).
The breadth of the first-order reflection peak may also be approximated using the following equation:Δg=(2/π)sin−1((nH−nL)/(nH+nL))  (equation 4)where Δg represents the ratio of half the peak width to the peak central wavelength, and nH and nL represent the refractive indices of the high and low refractive index layers, respectively. As can be seen, the greater the refractive index contrast, the greater will be the reflected peak width.
A MOF may also display higher-order reflections in different regions of the light spectrum than the first-order reflection. (See Radford, J. A. et al.). The wavelength of higher-order reflections can be approximated by the following:λX=(2/X)(n1d1+n2d2)  (equation 5)where X is an integer value, and λX is the Xth order of reflection.
For example, if a MOF is designed to have a first-order reflection peak at 1000 nm, it may also have higher-order reflection peaks at 500 nm (1000/2), 333 nm (1000/3), 250 nm (1000/4), and so on. One or more of these higher-order reflection peaks may be in the visible range, and would be perceived by an observer as a color reflection. If the visible region contains more than one reflection peak, the user may perceive a color which is a color mixture.
The relative intensities of the various orders of reflection is strongly dependent on the ratio of optical thicknesses (f-ratio) (see equation 3, and FIG. 2, Radford, J. A. et al.), given by:f=(n1d1)/(n1d1+n2d2)  (equation 6)
In the case of the first-order peak, the maximum reflection intensity is maximal at a f-ratio of 0.5, where the optical thicknesses of both layers are equal, and where both thicknesses correspond to a quarter-wavelength of the wavelength of maximum reflection intensity. The reflection of the first-order peak can remain relatively high, even for f-ratios down to 0.25 and below. The higher-order reflection, however, show a more marked dependence on the f-ratio. For example, the second-order reflection is suppressed at f=0.5, but maximal at f=0.25, the third-order reflection is maximal at f=0.167 and f=0.5, but suppressed at f=0.33, and the fourth-order reflection is maximal at f=0.125 and f=0.375, and suppressed at f=0.25 and f=0.5.
The intensity and width of higher-order reflections in a MOF are also related to the refractive index contrast, as for the first-order reflection (equations 2-4), for a given f-ratio.
While with the described MOFs have included alternating layers and periodically repeating layers, many other configurations are possible. Layers could be single or multiple in number, can vary in thickness in a variety of ways (homogenous gradient, periodic or semi-periodic arrangements, aperiodic arrangements, and superstructures), can comprise two or more compositions with two or more refractive indices, and can have different layers arranged above, below, or within the MOF to tune the optical properties. The variety of possible optical structures and their uses are well known, as described in (Macleod, H. A.; Thin-film optical filters, 3rd edition, Institute of Physics Publishing, 641 p.).
The positions of maximal reflection intensity for both the first-order and higher-order reflection peaks from a MOF show a dependence on the angle of the incident light. Equations 1 and 5 as noted above correspond to the wavelength reflected by light impinging normal to the multilayer surface (perpendicular to the layer direction). As the angle of incident light deviates from the perpendicular direction, the reflected wavelengths will blue-shift, or shift to lower wavelengths, a known property of such materials. (see Pfaff G., Reynders, P., Angle-dependent optical effects deriving from submicron structures of films and pigments, Chemical Reviews, 1999, 99, 1963-1981; Macleod, H. A.; Thin-film optical filters, 3rd edition, Institute of Physics Publishing, 641 p.)).
As can be seen from equations 1-6, the optical properties (including but not limited to the position, breadth, intensity and angle-dependence of both first-order and higher-order reflection peaks) of MOFs are heavily dependent on the refractive index and geometric thickness of the layers of the MOF. Consequently, MOFs that change optical properties in response to a thermal input (i.e., thermally tunable MOFs) may be created if the refractive index and geometric thickness of the layers of the MOF change in response to a thermal input.
Importantly, and as also demonstrated by equations 1-6, the greater the potential change in the refractive index and geometric thickness of the layers of the MOF, the greater the potential change in the MOF's optical properties. A larger potential change in the MOF's optical properties (i.e., a greater tunability) is desirable, because it allows a single MOF to provide a wider range of appearances, thereby enhancing the MOF's suitability for applications like security laminates, tax and excise stamps, machine-readable features, banknote foils, consumer product branding and brand product protection, and other printing and customization applications.
While the prior art provides some means to create tunable MOFs (see for example, WO 2009/143625, the entirety of which is incorporated herein by reference), there remains a need for MOF materials showing superior thermal tuning properties. Specifically, there is a need for a material whose reflected color can be broadly and irreversibly shifted in order to provide a permanent indication of instantaneous or cumulative heat load. There is also a need for such materials to have a continuous tuning range, such that multiple color states, corresponding to multiple conditions of time/temperature, may be generated.
There also exists the need for bright, thermally responsive materials with wide color tuning range in the fields of imaging and printing, specifically using direct thermal printers or laser writing/marking equipment.
Finally, there exists a need for improved color-shifting films and pigments and processes for tuning their colors. The color-shifting properties of MOFs make them candidates for color-shifting films and pigments, which are used widely in currency, ID documents, and as security features in other products. Color-shift is also used in a variety of non-security uses for aesthetic and eye-catching effects. Some color-shifting pigments can be printed by screen printing, which may increase the security of the color-shift effects. However, screen-printing cannot provide printing on-demand of arbitrary graphics or patterns. In the same way, more than one color of color-shift pigment may be printed in separate steps on the same substrate using screen printing, but high alignment accuracy between the printing steps is necessary. It would be highly beneficial to have a method to easily pattern color-shift materials on-demand in arbitrary patterns, and in multiple colors, using a single patterning step. It would be further beneficial to achieve this while not requiring consumables or multiple inks.
Past thermally tunable MOFs have had complicated manufacturing requirements. For example, the thermal tunability of the MOFs disclosed in WO 2012/162805 (referred to in that reference as Bragg Stacks) arises from a collapse in the porosity of its constituent layers. These porous layers are created by providing a layer comprising a polymer-particle composite, and subsequently etching away the particle with a chemical etchant.